In general, sewage may contain long fibrous refuse, volumes of large solids and the like. In a sewage pump for pumping such sewage, the refuse sometimes gets tangled in a suction end of an impeller or stuck between a blade of the impeller and a casing liner. The refuse gradually increases over time and clogs a channel in the impeller. The clogging often decreases the flow rate of the sewage transferred by the sewage pump.
This can be considered attributable to urbanization of recent years. Namely, the development of urbanization discourages the construction of sewage treatment plants in residential areas, but instead contributes the construction of intermediary pump stations in residential areas, which transfer water to suburban sewage treatment plants. To avoid the environmental pollution associated with bad smell coming from refuse and refuse transport, these pump stations barely remove a number of different waste materials mixed into sewage and transfer the sewage and the refuse together to the sewage treatment plants. This incurs clogging troubles in sewage pumps as mentioned above.
Sewage pumps for pumping sewage containing foreign objects, such as long fibrous refuse and volumes of large solids, drainage water containing garbage from kitchens, and the like, have been equipped with an open impeller or semi-open impeller to prevent the foreign objects from getting stuck in the impeller or the casing. However, the foreign objects still sometimes get stuck between the blade and the casing liner, hampering the pumping and incurring burnout in a drive motor or the like.
One proposed means for solving the above problems is a casing liner with radial grooves which is installed to face an impeller (FIGS. 2 and 3 of the Japanese Utility Model Public Disclosure No. S49-108103, and FIGS. 2 and 3 of the Japanese Utility Model Public Disclosure No. S64-11390). The means is to shred the fibrous foreign substances sucked in from the suction inlet of a sewage pump, by using the grooves of the casing liner and the impeller, and then discharge the foreign substances toward the outlet end of the pump. To be more specific, when foreign objects enter the radial grooves formed in the inner wall surface of the casing liner, blades shred the foreign objects as cutter blades in the position facing the casing liner to prevent the foreign objects from clogging in gaps. It is also suggested to form a groove that varies in depth in the surface of a pump housing, which faces the blade of a pump impeller (FIG. 4 of the Japanese Patent Public Disclosure No. H11-201087).